The present invention relates to a valve timing control system for controlling open/close timing of an intake valve and an exhaust valve of an internal combustion engine.
The valve timing control system comprises a driving rotator rotated by a crankshaft and a driven rotator integrated with a camshaft and mounted to the driving rotator so as to produce relative rotation as required. The mounting angle between the driving rotator and the driven rotator is appropriately controlled by mounting-angle changing means comprising a hydraulic actuator.
Typically, the valve timing control system controls lift timing of the engine valve at engine start to the most-lagged-angle side or to the most-advanced-angle side. Recently, study is made to use timing more outward of lift timing at engine start, i.e. timing on the most-lagged-angle side or on the most-advanced-angle side, in accordance with vehicle cruising conditions. In this case, lift timing at engine start, i.e. lift timing which allows engine start, is of necessity timing between the most lagged angle and the most advanced angle. Therefore, at engine start, the mounting-angle changing means need to return the mounting angle to a middle position, i.e. position between a most-lagged-angle position and a most-advanced-angle position.
In order to cope with such technical challenge, JP-A 2002-155714 proposes a valve timing control system which comprises a lock claw provided to one of the driving rotator and the drive rotator in a protrudable and withdrawable way and a recess formed in another and engageable with the lock claw at the middle position. The lock claw is biased by spring means in the lock direction, i.e. direction to be engaged in the recess. During lock released, the lock claw undergoes hydraulic pressure against the spring means.
With this valve timing control system, during ordinary engine operation, the release pressure is applied to the lock claw to allow free change of the mounting angle. At engine stop, the spring means press the lock claw forward with a reduction in hydraulic pressure. And when the mounting angle becomes at the middle position until the engine stops completely, the lock claw is then engaged in the recess. When being not engaged in the recess at engine stop, the lock claw is engaged therein when the driven rotator is fluttered by alternate torque of the camshaft, i.e. varying torque due to biasing force of a valve spring and a profile of a driving cam, during cranking at engine restart.